The present invention relates generally to improving the quality of the display provided by a liquid crystal display (LCD) screen. More particularly, the invention relates to techniques for analyzing and orienting display screens for their intended use in an actual product environment so as to provide a good visual quality for an observer viewing the screen at an expected angle when the product is in normal use.
Liquid crystal display screens are used in many applications and provide significant advantages such as thinness, low power consumption and low heat generation. They are frequently used in point of sale systems, in which efficient use of space and power is often highly beneficial. One notable characteristic of LCD display screens, which often introduces difficulties with their use, is that many characteristics influencing the quality of the display produced by a screen are highly dependent on the perspective of the observer with respect to the screen. A number of characteristics influence display quality, however, several are particularly important. Among the more important characteristics are luminance, that is, the physical correlate of perceived brightness, luminance contrast and color contrast. Moreover, these characteristics are highly dependent on the perspective of the observer. If a display as viewed from an observer""s perspective provides high luminance, high luminance contrast and high color contrast, the display tends to have high levels of legibility and image clarity when viewed from that perspective.
The perspective of the observer may suitably be described in terms of a skew angle and a roll angle. The skew angle is the angle between the observer""s line of sight to the screen and the normal vector, which is a vector perpendicular to the surface of the screen and extending out of the screen. The roll angle describes the orientation of the screen with respect to an observer looking in at the screen from the edge of the screen, and with the orientation being confined to the plane of the screen. For example, for a particular frame of reference, a roll angle of zero degrees, or 12 o""clock, may describe an orientation of the screen such that an observer is looking straight down at the edge of the screen from the top. A roll angle of 90 degrees, or 3 o""clock, would describe an orientation of the screen such that an observer is looking at the edge of the screen from the right side. A roll angle of 180 degrees, or 6 o""clock, would describe an orientation of the screen such that an observer is looking at the edge of the screen from the bottom, and a roll angle of 270 degrees, or 9 o""clock, would describe an orientation of the screen such that an observer is looking at the edge of the screen from the left side.
As noted above, luminance and contrast vary significantly with the perspective of the observer. This variation makes useful a concept called threshold viewing angle. Threshold viewing angle is the largest skew angle for a given roll angle at which a display is considered readable. For example, suppose an observer is looking downward at a display at a roll angle of 0 degrees, or 12 o""clock. In such a case, an exemplary display may provide acceptable image quality through a range of skew angles from 0 to 25 degrees. The threshold viewing angle at a 0 degree roll angle would therefore be 25 degrees. If the skew angle component of the line of sight of the observer is between 0 degrees and this threshold skew angle, the luminance contrast and brightness will be such that the display is legible. If the observer moves so that his or her line of sight falls outside of this threshold angle, the display will be unreadable because of poor luminance contrast and brightness.
Devices employing LCD screens are typically designed and built using standardized LCD screens which are manufactured by vendors and supplied to purchasers and incorporated into the devices designed by the purchasers. A purchaser typically selects a screen from among those offered by the manufacturer, rather than ordering a screen designed to the purchaser""s specifications, because it is not economical for a manufacturer to design a screen to order unless the quantity ordered is very great. Frequently, available LCD screens provide relatively poor display quality when an observer looks downward at the screen, that is, at a relatively large skew angle at a 12 o""clock roll angle, or upward at the screen, that is, at a relatively large skew angle at a 6 o""clock roll angle, compared to looking from the left or right, that is, at a large skew angle at a 9 or 3 o""clock roll angle. These differences in visual quality are related to differences in luminance contrast and brightness. Moreover, there are typically differences in quality between upward and downward viewing angles. Which orientation is better depends to some degree on the photometric characteristics of the displayed images at 0 degrees skew angle. Text screens composed of black lettering on a white background will often look better at 6 o""clock than at 12 o""clock because the luminance contrast is typically better at 6 o""clock. On the other hand, very dark images might look worse at 6 o""clock because image brightness is often substantially reduced at these angles.
In a point of sale terminal designed for a standing user for example, the user looks downward at the display. As noted above, however, many LCD screens supplied by manufacturers are designed such that the screen provides a relatively poor visual quality, and a reduced threshold viewing angle, for an observer looking downward at the screen. It is possible to design a terminal having the screen tilted upward so that the observer views the screen at a reduced skew angle, that is, at an angle more nearly normal to the screen, so that the detrimental effect of the reduced threshold viewing angle is reduced. However, in the commonly encountered situation in which lights are mounted in the ceiling of a room so that the ambient light emanates from above, angling the screen upward contributes to glare and decreases readability.
There exists therefore, a need for techniques for incorporating an LCD screen into devices which will allow the screen to be oriented in such a way as to provide the maximum possible display quality for the perspective from which the screen is most likely to be viewed.
According to one aspect, the present invention comprises a point of sale terminal including an LCD screen oriented so as to provide an improved display quality and threshold viewing angle for an observer looking downward at the screen. The point of sale terminal may suitably employ a standard LCD screen which provides an improved display quality to an observer looking upward at the screen when the screen is mounted in a conventional configuration. In order to provide a superior display quality for an observer looking downward, the screen is mounted in an orientation rotated 180 degrees from the conventional configuration. In order to prevent the displayed material from being presented upside down, display adjustment software is used to rotate the displayed material 180 degrees. The rotation of the display on the screen, combined with the physical rotation of the screen, causes the display to be correctly presented to an observer. The display adjustment software may be permanently resident, for example in ROM, or may be loaded automatically at initialization of the terminal.
An alternative embodiment of the present invention comprises a point of sale terminal including an LCD screen oriented so as to provide an improved display quality and threshold viewing angle for an observer looking downward at the screen, and employing an LCD screen which provides an improved display for an observer viewing from the left or right of the screen, that is, at a 9 or 3 o""clock roll angle, when the screen is mounted in its conventional configuration. In this embodiment, the screen is mounted in a configuration rotated 90 degrees left or right from the conventional configuration and display adjustment software is used to rotate the displayed material 90 degrees left or right, as required, so that the display is correctly presented to an observer.
A more complete understanding of the present invention, as well as further features and advantages of the invention, will be apparent from the following Detailed Description and the accompanying drawings.